text: An injection molded plastic part is considered "safe" to remove from its mold once it is near or below the HDT. This means that part deformation will be held within acceptable limits after removal.  The molding of plastics by necessity occurs at high temperatures (routinely 200 °C or higher) due to the low viscosity of plastics in fluid form (this issue can be addressed to some extent by the addition of plasticizers to the melt, which is a secondary function of a plasticizer). Once plastic is in the mold, it must be cooled to a temperature to which little or no dimensional change will occur after removal.  In general, plastics do not conduct heat well and so will take quite a while to cool to room temperature. One way to mitigate this is to use a cold mold (thereby increasing heat loss from the part). Even so, the cooling of the part to room temperature can limit the mass production of parts.
  Choosing a resin with a higher heat deflection temperature (and therefore closer to melting temperature) can allow manufacturers to achieve a much faster molding process than they would otherwise while maintaining dimensional changes within certain limits.